1. Field of the Invention
The present invention generally pertains to the art of active electronic devices and, in particular, electronic amplifiers which produce both voltage noise and current noise. More particularly, the invention relates to an amplifier circuit for reducing the effect of current noise at the amplifier input while also reducing the voltage noise in most practical cases.
2. Discussion of the Prior Art
Measurement and amplification systems are sometimes limited by the effect of their internal current noise acting on the impedance of an external element connected to the amplification system. This current noise is multiplied by the impedance of the element to give an effective voltage noise at the amplifier input. In an arrangement where an amplifier is connected to a very high impedance antenna, such as a pure capacitor, there will be a frequency below which the voltage noise referred to the system input is dominated by the effect of current noise acting on the antenna impedance. Essentially, as active electronic devices and, in particular, electronic amplifiers, produce both voltage noise and current noise, it is desirable to have some type of noise reduction system to make these devices more sensitive.
One proposed way of reducing the effect of voltage noise is to use the well-known method of chopping an input voltage signal. Such chopper stabilization was developed around 1950 and is currently the standard technique employed in low DC offset operational amplifiers. Referring to FIG. 9, there is shown an overall circuit using the chopper stabilization approach. A signal voltage source 1015 produces a signal VIN which is sent to an amplifier 1030. An output 1032 from amplifier 1030 connects to an AC gain amplifier 1033. An output of AC gain amplifier 1033 is connected with a demodulator 1040 which ultimately produces an output signal VOUT at output 1047. A modulation signal frequency generator 1050 is provided to send a synchronizing signal 1055 to demodulator 1040. Also, modulation signal frequency generator 1050 controls a switch 1062, while a ground is established at 1072. Switch 1062 ultimately sends voltage signal VIN to either a non-inverting terminal 1082 of amplifier 1030 or an inverting terminal 1083 of amplifier 1030 at the modulation signal frequency. The circuit switches voltage signal VIN at amplifier 1030 in order to reverse the sign of the signal presented at the amplifier input. DC input voltage signal VIN is transferred into an amplified AC signal Vamp, which can then be further amplified as an AC signal. Demodulator 1040 is provided to synchronously demodulate the signal back to a DC signal VOUT at output 1047.
Every amplifier has a DC offset or offset drift. In this case, the DC offset or offset drift is referenced as Voff. With the voltage at amplifier output 1032, Vamp, being a square wave signal which swings between VIN+Voff and −VIN+Voff, any voltage due to the amplifier's offset drift, which essentially is a form of noise, has no effect on the peak to peak amplitude of the square wave which is given by 2VIN. If the input signal is not a DC signal but still alternates at a relatively low frequency, fs, chopping generates sidebands about the chopping frequency, fmod, occurring at fmod±fs. Converting the signal to a higher frequency is of benefit because, typically, the voltage noise of almost all amplifiers decreases with increasing frequency of the input signal.
Another simple circuit to illustrate chopper stabilization for an amplifier 1030 with voltage source 1015 is shown in FIG. 10. Voltage source 1015 is connected through a capacitor 1020 to amplifier 1030 which provides an output at 1032. A switch 1063 is provided between capacitor 1020 and the non-inverting terminal 1082 of amplifier 1030. Inverting terminal 1083 is sent to ground 1072. The capacitance Camp of amplifier 1030 is also shown going to ground 1072. Finally, a voltage noise source 1080 and a current noise source 1085 is illustrated.
While the technique of chopper stabilization is well established and is effective at reducing an amplifier's voltage noise, it is ineffective in situations where the amplifier input noise is dominated by the amplifier's current noise. The circuit shown in FIG. 10 shows a situation where current noise IN is likely to be a limiting factor in the overall sensitivity of the device. An input signal, VIN, is detected by antenna or voltage source 1015 in which impedances are represented by capacitance Cs. Capacitor 1020, having a capacitance Cs, is connected to amplifier 1030 with a gain of 1 and with input capacitance Camp. Of course the gain could, be some other value besides 1.In any case, an input current noise, IN, and a voltage noise, Vn, are also present as capacitor 1020 is connected through switch 1063. Chopper stabilization can be achieved by simply opening and closing switch 1063 at a certain frequency, fmod, making amplifier 1030 insensitive and sensitive to signal VIN respectfully. If the circuit transfer function from the signal voltage to the amplifier is defined as Xsig in the sensitive state, the voltage at output 1032 of amplifier 1030 is given by:
                              V                      amp            ,            sens                          =                                            X              sig                        ⁢                          V                              I                ⁢                                                                  ⁢                N                                              +                                    I              N                        ⁢                          Z                              i                ⁢                                                                  ⁢                n                                              +                      V            N                                                  =                                                            C                s                                                              C                  s                                +                                  C                  amp                                                      ⁢                          V                              I                ⁢                                                                  ⁢                N                                              +                                    I              N                                      ω              ⁢                                                          ⁢                              (                                                      C                    s                                    +                                      C                    amp                                                  )                                              +                      V            N                              and in the insensitive state by:
      V          amp      ,      insens        =                    I        N                    ω        ⁢                                  ⁢                  C          amp                      +          V      N      
With this in mind, at non-inverting terminal 1082 of amplifier 1039, the signal amplitude is multiplied by a square wave going between 0 and 1 at a frequency fmod. The current is multiplied by a square wave going between
      1          ω      ⁡              (                              C            S                    +                      C            amp                          )              ⁢          ⁢  and  ⁢          ⁢      1          ω      ⁢                          ⁢              C        amp            at frequency fmod, and the voltage noise is constant. The fact that the term which represents current noise is modulated at frequency fmod means that it is converted along with the signal and, thus, will not be eliminated or reduced.
Despite the above-discussed proposed solution to reduce voltage noise in an electronic amplifier, none of these methods have been successful at reducing current noise. Neither the traditional method of chopper stabilization nor the prior techniques of capacitive modulation offer effective reduction of current noise. Therefore, there exists a need in the art to provide an amplifier circuit with both voltage and current noise reduction.